An approximation for hindered rotor state energies

A method suggested by Troe [J. Chem. Phys. 66 (1977) 4758] for estimating hindered internal rotation state energies is first evaluated and then extended by including anharmonic terms and introducing a smooth switching function. Comparisons between calculated thermodynamic functions and exact results [J. Chem. Phys. 10 (1942) 428] show that the extended method is greatly improved and performs at a level comparable to other approximate methods, but without their drawbacks. The extended method is very useful for calculating the densities of states and partition functions needed for reaction rate theories.